Synchronization channels are mainly used for cell search purposes. Typically this includes primarily initial cell search, and possibly handover measurements.
For initial cell search, the user looks for a known sequence (a Primary Synchronization Channel (P-SCH)) and tries to identify the carrier and frame/symbol timing from it. Cell search methodology then proceeds by identifying the identity of the cell, possibly using a Secondary Synchronization Channel S-SCH and a cell specific scrambling/spreading/pilot code, i.e., a common pilot channel. The P-SCH is transmitted in a similar form from all (or at least multiple) cells in the system, and by receiving the P-SCH over the P-SCH cycle, an estimate of the signal strength of all cells transmitting in the same band can be acquired. After synchronizing to the downlink transmission and possibly acquiring a channel estimate for at least part of the bandwidth, the user may decode a part of a broadcast channel and may learn system information that is required to perform random access in the uplink and to read the downlink traffic channels and the control channels associated with the traffic channels.
The users in an active state are typically not interested in receiving the SCH of the serving cell. They perform fine synchronization and estimate the channel to the serving cell directly from the cell specific pilots. When performing handover measurements, the user equipment (UE) may know the identities of neighboring cells, so the UE may start directly looking for pilot codes of the neighbors. It may be more effective, however, to search for strongest neighbors by running cell search algorithms, looking for the P-SCHs of other cells. In an asynchronous network other cells' SCHs are not to be found at the same timing as the SCHs in the serving cell. From the resource allocation point of view, it is likely that active state UEs follow common control channels (CCCH), on which, e.g., an Allocation Table is transmitted. This allocation table divides resources, e.g., on a Downlink Shared Channel (DSCH).
The radio transmission may be divided into radio frames, and the radio frames to sub-frames. For the purposes of this invention, a frame is a time period during which the SCHs are transmitted, and a sub-frame is a time period during which the resources are allocated by the CCCH. There are multiple sub-frames in a frame. A normal sub-frame could be, for example, such that there are pilots and the CCCH in the first OFDM-symbol, and the six consecutive OFDM-symbols are the DSCH.
There can be a predefined principle by which the CCCH addresses resources in the DSCH. For this, e.g., the resources in the DSCH are divided into resource units, which may be separately allocated. In FIG. 1, a DL (downlink) EUTRA (evolved universal terrestrial radio access) sub-frame is depicted with resource units and shared/control/pilot channel multiplexing.
In FIG. 1, there are seven OFDM (orthogonal frequency division multiplexing) symbols in a sub-frame. As an example, the first symbol is reserved for the common pilot channel and the common control channel. The remaining six symbols are dedicated to the downlink shared channel. FIG. 1 shows four different ways (examples) of dividing the DSCH resources to resource units as depicted. The uppermost variant is pure frequency division multiplexing (FDM). A number of sub-carriers during the whole duration of the DSCH belong to the resource unit. The other variants involve different degrees of time division multiplexing (TDM) in addition to the FDM. The sensible TDM units are comprised of 6, 3, 2, and 1 OFDM symbols. It is shown pictorially that depending on the degree of applying the TDM, the resource unit in a frequency domain may be smaller or bigger, so that the total resource unit is approximately of the same size. The size of the resource unit is determined by two competing principles. First, there is a limited addressing space, set by the limited size of the CCCH, favouring larger resource units. There is however a need for a specified minimum resource unit size.
Tentatively, it is expected to be 75, 150, 300, 600, 900 and 1200 subcarriers in an DL EUTRAN OFDM symbol on 1.25, 2.5, 5, 10, 15 and 20 MHz channel bandwidths, respectively. Possible alternatives for the resource unit size in the frequency domain (i.e., reflecting a number of subcarriers or a width of the resource unit in a frequency domain) that have been discussed are, for example, 12, 25, 30, and 50. The two latter would not fit into the 1.25 MHz bandwidth alternative, whereas the first would not fit into the two narrowest bandwidth alternatives. In case of a resource unit size of 12 subcarriers, the number of subcarriers in the narrower bandwidth alternatives may be reduced. Alternatively, some resource units may be defined to be larger, or smaller.
In addition to the rectangular resource units depicted in FIG. 1, distributed resource units have been discussed. These are such that the sub-carriers of the resource unit are not necessarily all neighbors of each other. This way, there is more frequency diversity in a resource unit. Such resource units may be described as in FIG. 1, if the sub-carriers in the resource allocation are mapped to the true sub-carriers used for transmission by a permutation. In addition, a permutation may operate in different ways on different OFDM-symbols, resulting in arbitrarily complex sets of resource units.